Agent API and payments glossary

x402 lets an API charge per request over HTTP with a stablecoin payment; MCP (Model Context Protocol) lets AI agents discover and call tools. FastAF supports both.

• x402: an open payment protocol that uses HTTP status 402 so a client can pay for a request and retry it with proof of payment. FastAF supports x402 v2.
• HTTP 402 Payment Required: the status code an x402 server returns with payment requirements (price, asset, network, payTo).
• Facilitator: a service that verifies and settles x402 payments on-chain for the server.
• USDC on Base: the stablecoin and network FastAF accepts for x402 payments.
• Bazaar: x402 discovery metadata that lets agents find paid endpoints and their prices; FastAF publishes it.
• Pay-per-call: paying for each request individually, without an account or subscription.
• MCP (Model Context Protocol): an open protocol for connecting AI clients to tools and data.
• MCP server: a service exposing tools over MCP. FastAF’s is at https://api.fastaf.co/mcp with 23 tools.
• Streamable HTTP: the remote MCP transport FastAF uses.
• MCP tool: a named function with an input schema, e.g. amazon_product, scrape_url.
• MCP registry: a directory of MCP servers; FastAF is listed in the official MCP Registry.
• OpenAPI: a machine-readable API description; FastAF’s is https://api.fastaf.co/openapi.json.
• SERP: search engine results page.

How do you make a real request?

This example calls Google SERP API using its documented input. Save the API key from the signup email, confirm your email and set FASTAF_API_KEY in your server environment.

curl --fail-with-body -H "Authorization: Bearer $FASTAF_API_KEY" 'https://api.fastaf.co/v1/serp?query=best+espresso+machine+for+home&country=us&language=en'

# pip install fastaf-sdk
import os
from fastaf import FastAF

with FastAF(os.environ["FASTAF_API_KEY"]) as client:
    data = client.request("serp", {"query":"best espresso machine for home","country":"us","language":"en"})
    print(data)

Google SERP JSON. organic_results contains ranked title, link and snippet fields; search_information describes the query. Other Google result blocks may also appear.

The example costs 5 credits after the trial. Alternatively, this endpoint costs $0.003 per successful x402 call at the default configuration; FastAF x402 calls start at $0.003. These are separate billing modes.

For x402, request the same parameters under /x402/v1/serp without an API key, validate the live PAYMENT-REQUIRED challenge, then sign and retry with a payment-aware client. A bare curl request returns HTTP 402 and does not pay; unsupported higher-cost options require API-key billing.

Frequently asked questions